1. Field of the Invention
Embodiments of the invention relate to a flash memory system. More particularly, embodiments of the invention relate to a flash memory system capable of compensating for reduced read margins between memory cell program states.
This U.S. non-provisional patent application claims priority under 35 U.S.C § 119 of Korean Patent Application 2006-07420 filed on Jan. 24, 2006, the entire contents of which are hereby incorporated by reference.
2. Discussion of Related Art
In recent years, storage devices such as volatile memory devices and non-volatile memory devices have been increasingly applied to MP3 players and mobile appliances such as, for example, portable multimedia players (PMPs), cellular phones, notebook computers, and personal digital assistances (PDAs). The MP3 players and the mobile appliances require mass storage devices for offering various functions (e.g., moving picture playback). Many efforts have been made for meeting the requirement. One of these efforts is to propose a multi-bit memory device where at least 2-bit data are stored in one memory cell. Exemplary multi-bit memory devices are disclosed, for example, in U.S. Pat. Nos. 6,122,188; 6,075,734; and 5,923,587 which are incorporated herein by reference.
When 1-bit data is stored in one memory cell, the memory cell has a threshold voltage belonging to one of two threshold voltage distributions, i.e., the memory cell has one of two states indicating data “0” and data “1”. On the other hand, when 2-bit data is stored in one memory cell, the memory cell has a threshold voltage belonging to one of four threshold voltage distributions, i.e., the memory cell has one of four states indicating data “11”, data “10”, data “00”, and data “01”. Threshold voltage distributions corresponding to four states are illustrated in FIG. 1.
Threshold voltage distributions corresponding to four states should be carefully controlled such that each of the threshold voltage distributions exists within a determined threshold voltage window. In order to achieve this, a programming method using an increment step pulse programming (ISPP) scheme has been suggested. In the ISPP scheme, a threshold voltage shifts by the increment of a program voltage according to the repetition of program loops. By setting the increment of a program voltage to a small value, threshold voltage distributions may be minutely controlled to secure a sufficient margin between states. Unfortunately, this leads to increase of time required for programming a memory cell to reach a desired state. Accordingly, the increment of the program voltage may be determined based on the programming time.
In spite of such an ISPP scheme, a threshold voltage distribution of each state is generated to be wider than a desired window due to various causes. For example, as indicated by dotted lines 10, 11, 12, and 13 of FIG. 1, a threshold voltage distribution is widened due to a coupling between adjacent memory cells in a programming operation. Such a coupling is called an “electric field coupling” or “F-poly coupling”. For example, as illustrated in FIG. 2, assuming that a memory cell MCA is a cell programmed to have one of four states and a memory cell MCB is a cell programmed to have one of four states, charges are accumulated in a floating gate (FG) as the memory cell MCB is programmed. When memory cell MCB is programmed, a voltage of floating gate FG of adjacent memory cell MCA rises due to a coupling between floating gates FG of the memory cells MCA and MCB. The rising threshold voltage is maintained due to a coupling between floating gates even after programming memory cell MCB. The memory cell MCB includes memory cells arranged in a wordline direction and/or a bitline direction relative to the memory cell MCA. Due to such a coupling, the threshold voltage of the programmed memory cell MCA rises and the threshold voltage distributions are widened as indicated by the dotted lines 10, 11, 12, and 12 of FIG. 1. Accordingly, a margin between states is reduced, as illustrated in FIG. 1 which is a reduction of the read margin (difference in voltage in determining the presence of a “1” or a “0”).
One conventional technique for preventing a threshold voltage distribution from being widened due to a coupling is disclosed in U.S. Pat. No. 5,867,429.
Not only an electric field coupling/F-poly coupling but also a read margin between states is reduced as threshold voltages of memory cells drop with the lapse of time, which will be hereinafter referred to as “hot temperature stress (HTS)”. HTS means that charges accumulated in a floating gate of a memory cell are drained to a substrate. As the charges of the floating gate are reduced, threshold voltages of memory cells in respective states drop, as indicated by dotted lines 20, 21, and 22 of FIG. 3. Accordingly, a threshold voltage increases due to an electric field coupling/F-poly coupling and a threshold voltage decreases due to HTS which makes it difficult to secure a read margin between states. In particular, it is difficult to know a state of the programmed memory cell. This problem becomes severe with the recent trend toward more complex semiconductor fabrication processes.
Accordingly, there is a need for securing a read margin between states even if a threshold voltage increases due to an electric field coupling/F-poly coupling and a threshold voltage decreases due to HTS.